1. Field of the Invention
The present invention relates generally to the field of liquid crystal displays including an LED (Light Emitting Diode) backlight. More particularly, the application relates to a liquid crystal display including a multimode LED backlight that is compatible with night vision imaging system (NVIS) requirements.
2. Description of the Related Art
Single mode NVIS (Night Vision Imaging System) compatible backlights are reliant on expensive and complex IR (Infra-red) filters with requisite engineered high optical transmission.
Material and labor associated with IR filtering solutions remain a top cost driver for avionics grade military AMLCD (Active Matrix Liquid Crystal Display) products. Most single mode military display designs utilize IR filtering via a fairly complex glass component with multiple thin film treatments or a laminated assembly of coated and/or absorptive filters. Further driving the cost and complexity (and value) of these filters is the need for high optical transmission in order to avoid generating higher secondary costs and complexity in the traditional single mode backlight (more power results in more LEDs).
Many previous dual mode backlights experienced complexity and cost creep which negated their basic virtues of day mode power reduction and large filter component cost reduction. Often, prior dual modes required custom waveguides and expensive fab parts to host and mount complex assemblies. Traditional dual mode backlights also had producibility issues in several areas as well. Multiple application, cleaning and cure cycles and the resultant labor associated with the use of adhesives used to prevent light leaks around the filters proved to be challenging and generated painfully long durations for work in process. Many prior dual modes used the same expensive NVIS filter components (just smaller, higher count) as their single mode backlight assembly counterparts, negating potential cost savings and resulting in the same limited supply source issues. In addition to high component material cost from the filters, the use of multiple PWBs (Printed Wiring Boards) and resultant needed interconnect also drove related material and labor costs higher. The complexity and cost of these prior dual mode backlight architectures has limited the ability to respond to increasing cost, producibility and performance pressures.
Cost reduction efforts for the current large multi-film IR filters have been fairly successful, reducing filter costs by as much as 60-70%. Yet these traditional NVIS filters remain as one of the highest cost reduction opportunities in existing display products for military applications.
Further significant cost reductions from traditional suppliers, however, seem unlikely. Cost creep upward may be a more likely scenario.
U.S. Pat. No. 6,419,372, entitled “Compact Optical Wave-Guide System for LED Backlighting Liquid Crystal Displays,” issued to J. E. Shaw et al., discloses a flat panel display and a backlight which provides both night vision and day mode backlighting of the flat panel display. The backlight of the flat panel display includes a circuit card and day mode light emitting diodes (LEDs) positioned coplanar with each other on the circuit card. Night Vision (NVIS) LEDs are positioned on the circuit card coplanar with the day mode LEDs. An optical wave-guide is positioned in front of the day mode LEDs and the NVIS LEDs and is adapted to distribute light from the day mode LEDs and from the NVIS LEDs forward away from the LEDs and the circuit card.
U.S. Pat. No. 6,789,921, entitled “Method and Apparatus for Backlighting a Dual Mode Liquid Crystal Display,” issued to C. T. Deloy et al., discloses an LED backlight system for dual mode operation with an NVIS system which utilizes non-edge lit night mode LED and a thermal conduction path from the day mode array of LEDS to a rearward disposed heat sink which cross the night mode array of LEDS. The system includes first and second LED arrays, the second array being located behind the first array. Light from the second LED array pass through a plurality of apertures located on the circuit card supporting the first array.
U.S. Pat. No. 6,842,204, entitled “Color Display System For NVIS Class A Compatibility,” issued to R. J. Johnson, discloses a full color liquid crystal display system providing compatibility with Class A night vision imaging goggles. One embodiment of the invention is directed to a color liquid crystal display (LCD) having dual modes of operation. The color LCD display includes a normal mode light source, a liquid crystal display stack positioned so as to receive light from the normal mode light source, and a night vision imaging system (NVIS) mode light source. The NVIS mode light source includes a NVIS mode white light source, a NVIS mode red light source, a NVIS filter receiving light from both the NVIS mode white light source and the NVIS mode red light source, and a waveguide receiving light from the NVIS filter for reflection to the LCD stack.
The introduction of LEDs as a practical light source for avionics backlighting has created novel opportunities to address classic NVIS filtering challenges and performance compromises that were present in traditional single mode backlights. The present invention, as disclosed below, represents a synthesized solution to these challenges.